Most likely following standard action gamer SOP -> top of after every wave.

In most action games the odds of running out of ammo on the next wave of baddies is much higher then killing them, reloading, and then getting shot. Especially when reload speed is done in that super fast method meant to simulate reloading in a firefight. It's kind of been that way since things went from total ammo counter to magazine/total ammo counters.